As electrophotographic photosensitive members used in electrophotographic image forming apparatuses, it is known to use inorganic photosensitive members having a photosensitive layer composed of an inorganic material, such as selenium and a-silicone, and organic photosensitive members having a photosensitive layer mainly composed of organic materials, such as a binder resin, a charge-generating material, and a charge transport material. Among these photosensitive members, organic photosensitive members have been widely used because of the ease of production compared with inorganic photosensitive members, wider selectivity of materials for the photosensitive layer, and higher design freedom.
An example of such organic photosensitive members is a single-layer-type organic photosensitive member which has a photosensitive layer including at least a charge-generating material and a charge transport material within the same layer. As compared with a multilayer-type organic photosensitive member in which a charge generation layer containing at least a charge-generating material and a charge transport layer containing a charge transport material are stacked on an electrically conductive substrate, the single-layer-type organic photosensitive member is advantageous in that it has a simple structure, is easy to manufacture, and can suppress the occurrence of film defects, and thus it is widely used.
In recent years, the size of multi function peripherals and printers has been reduced and the printing speed has been increased. Accordingly, photosensitive members which are to be used in multi function peripherals and printers capable of high-speed printing are required to have higher sensitivity so as to be able to print with the high-speed process.
Conventionally, a metal-free phthalocyanine is used as a charge-generating material of the single-layer photosensitive member. However, there is a limit in achieving higher sensitivity. In contrast, an oxotitanium phthalocyanine has a higher quantum efficiency than the metal-free phthalocyanine and is a charge-generating material that is very useful in increasing the sensitivity of an electrophotographic photosensitive member.
However, when oxotitanium phthalocyanine is used in a high-speed process, charge acceptance of the electrophotographic photosensitive member degrades after repeated use, and fog, black stripes, density unevenness, and the like occurs in the resulting image. The reason for this is believed to be that, although advantages, such as high responsiveness, are achieved because the high sensitivity property of oxotitanium phthalocyanine causes a relatively large amount of charge generation, when oxotitanium phthalocyanine is used in the high-speed processes, a memory phenomenon occurs wherein a charge remains in the photosensitive layer and the difference in potential between the exposed portion and the non-exposed portion decreases.
In order to prevent the occurrence of the memory phenomenon, for example, an oxotitanium phthalocyanine and another phthalocyanine may be combined, or an oxotitanium phthalocyanine having a maximum peak at a Bragg angle (2θ±0.2°) of 27.2° in an X-ray diffraction spectrum and a charge transport agent may be incorporated into a photosensitive layer.